The xXPSx models use a transverse mounted battery (CR123A), otherwise they are essentially the same as the long-nose. Some of the long-nose models like the 556 use the same CR123A battery that the xXPSx models do, so no real differences in overall sight function and electronics or battery life.
CR123A batteries can be bought in bulk for cheap from Amazon or eBay, and afaik they have a longer shelf-life than AA alkalines.
The advantages are:
- about a 2" shorter length, good if you have a short rail to mount it to.
- different battery cap seal, supposed to be better according to EOTech.
- battery cap has a cable holding it to the base so you don't drop/lose it.
The disadvantage is that batteries don't take a lot of sideways shock, so you might find your battery warped and difficult to remove or even leaking.
Some people freak out over this, I'm not worried because I check all of my batteries in all of my gear every month or two just to make sure nothing is leaking in any of them anyway.
Personally I prefer the looks of the shorter models, and it fits better with no overhang on my rifle which has a short top rail.
I wish that EOTech actually made a "mini" version of the sights, that was about 3/4 the size, since these sights are rather bulky.
